Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a jib connection structure composed of a jib connection shaft projecting in both lateral directions of a boom leading end and jib base end engagement portions provided at a bifurcated jib base end. The jib base end engagement portions are U-shaped and can be engaged with the jib connection shaft. The jib base end engagement portions can be connected to the jib connection shaft by engaging the jib base end engagement portions with the jib connection shaft and inserting pins into through holes in leading ends of the jib base end engagement portions.
For a jib projection operation, a jib is firstly placed in a lower holding position of a boom and the jib base end engagement portions are connected to the jib connection shaft. At this time, the jib is placed at a slight angle with respect to the boom (“offset placement”). One of the jib base end engagement portions is formed to be longer than the other to allow the pins to be inserted into the through holes in the jib base end engagement portions while the jib base end engagement portions remain in the offset placement. Subsequently, the boom is elevated up, so that the jib is suspended from the boom leading end. Lastly, the jib is projected, by exerting tension on a tension rod. A jib stowage operation is done in a procedure reverse to the jib projection operation.
The jib base end engagement portions on the right and left have longitudinal dimensions different from each other, and the pins on the right and left to be inserted into the jib base end engagement portions are also located at different positions. Accordingly, in a state where the jib is suspended from the boom leading end, only one of the pins is locked to the jib connection shaft, and the jib is suspended at this one point. Consequently, there is a problem that the jib swings sideways and is not stabilized, so that it is difficult to carry out the jib projection/stowage operation.